The report suggests that drinking 200-300mg of caffeine per day raised the risk of a baby being born small for gestational age by between 27% and 62%.

Smaller babies are at higher risk of certain health problems and the researchers say recommendations on safe limits need to be reconsidered.

UK guidelines advise pregnant women to limit caffeine intake to no more than 200mg per day.

A spokesperson for the Royal College of Obstetricians and Gynaecologists says it merits a wider look at the evidence to see if the current guidelines needed to be revised.

Baby gets some of what mum eats and drinks

Everything the mum-to-be eats and drinks potentially gets through to the growing baby. That's why health organisations like the NHS set recommendations for limits on things like alcohol and caffeine in pregnancy.

Along with nutrients and oxygen, caffeine passes from the placenta to the baby. However, the effect isn't the same on a baby as it is in adults.

Dr Pat O'Brien is a spokesperson for the Royal College of Obstetricians and Gynaecologists. "Does it make the baby buzz? We don't know. It's been shown that it crosses the placenta and it does affect blood vessels. You can expect it to affect blood flows in the baby and the placenta.

"People haven't seen a significant effect on heart monitoring after caffeine intake."

Caffeine recommended limits

The World Health Organisation suggests a caffeine limit of 300mg per day during pregnancy. The NHS recommendation is lower at 200mg.

One mug of filter coffee contains around 140mg of caffeine. Caffeine levels may be stronger in shop bought coffee.

For the new research published in the journal BMC Medicine, researchers looked at information about diets and birth details of around 60,000 pregnancies over 10 years.

All sources of caffeine were monitored in the study.

Study results

In a news release, Dr Verena Sengpiel, from Sahlgrenska University Hospital in Sweden who led the project says: "In this study we found no association between either total caffeine or coffee caffeine and preterm delivery, but we did find an association between caffeine and SGA [small for gestational age]."

Mothers to be who had 100mg of caffeine from all sources a day reduced their babies birth weight by around 21-28g overall.

The source of caffeine also seemed to make a difference. Women who daily had 100mg of caffeine in general increased the length of their pregnancy by five hours. However, caffeine from coffee was found to add eight hours overall from drinking 100mg a day.

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